The purpose of this project is to study the effects of an exercise challenge and exposure to allergen on the symptoms of CFS. Our aim is to induce the symptoms of CFS with these challenges and record any abnormalities in the immune response over 24 hours. As of February 1, 2000, 54 total subjects have been enrolled in the study. 35 CFS subjects and 19 control subjects. CFS 35 31F 4M(Average age 40 y) Control 19 11F 8M(Average age 31 y) The recruitment of new control subjects is going to be limited to ages 30-45 to increase the average age within that group. To date the following challenges have been completed: Exercise 16 CFS 15 Control Total: 31 Histamine 14 CFS 13 Control Total: 27 Allergen 8 CFS 8 Control Total: 16